Structural element with anticipated prestressing

ABSTRACT

The invention provides a prefabricated structural element including an elongate body ( 11 ) and at least one first tensioner ( 1 ) that is fastened in the elongate body in such a manner that it compresses the elongate body. The structural element includes at least one second tensioner ( 2 ) that is fastened to the elongate body at two distinct points by two fastener element so that it compresses the elongate body, at least one of the fastener element being removable so as to make it possible to relax the compression exerted on the elongate body by the second tensioner.

TECHNICAL FIELD TO WHICH THE INVENTION RELATES

The present invention generally relates to fabricating structural elements, in particular concrete beams, slabs, or blocks.

It relates more particularly to a prefabricated structural element comprising an elongate body and at least one first tensioner that is fastened in the elongate body in such a manner that it compresses and bends the elongate body in a first direction.

It also relates to a method of installing such a structural element in a work.

The invention applies to any type of work, e.g. buildings, bridges, dams, . . . .

TECHNOLOGICAL BACKGROUND

During the building of such a work, it is common practice to use concrete beams and slabs.

A well-known problem of concrete is that although it is good at withstanding compression forces, it cracks quickly when it is subjected to traction forces. Thus, it is estimated that concrete withstands compression forces twenty times better than traction forces.

It is thus known to reinforce concrete with metal reinforcement. This is referred to as reinforced concrete.

However, although reinforced concrete presents certain advantages, its use becomes counter-productive when the stresses exerted on the structure become large, as a result of using reinforced concrete of greater section.

The solution that is then envisaged is to use “prestressed” concrete.

The idea is thus to ensure that the concrete always works in compression and never (or hardly) in traction. To do this, tensioners (generally steel bars or metal cables) are placed in the concrete and traction is exerted on them such that the concrete is compressed while at rest.

In this way, when the concrete is subjected to traction forces, it decompresses, but it never works in traction, thereby avoiding cracks appearing.

Currently, two methods of fabricating prestressed concrete beams are known.

The first method is pre-tensioning and consists in applying a tension to the tensioners before the concrete has fully set. After the concrete has cured, the tensioners are released, thus compressing the beam merely by adhering thereto.

That method is simple to implement. Generally, when the beam is designed to be placed horizontally and to be loaded, the tensioners are off-center relative to the neutral axis of the beam so that they make it possible for the beam to bend upwards (referred to as “hogging”), against the load that the beam is to receive (once loaded, the beam is thus deformed a little).

However, that method presents two drawbacks.

The first drawback is that when unmolding the beam and releasing the tensioners, said tensioners cause excessive forces on the beam that cause the beam to bend quickly, with the risk of such traction or compression forces generating cracks in the concrete. It is thus necessary to wait for the concrete to be thoroughly cured before relaxing the tensioners and unmolding the beam.

Another drawback is that when the beam is not yet loaded, its bending generates traction stresses in the concrete (at the convex face of the beam), and particularly strong localized compression stresses (at the concave face of the beam). As a result, the prestress generated by the first tensioners must not exceed a threshold beyond which the beam bends so much that it cracks.

The second method is post-tensioning and consists in putting tensioners through straight or curved sheaths that are incorporated in the concrete of the beams. After the concrete has set, the beam is put into place in the work (building, bridge, . . . ) before the tensioners are tensioned. Once the beam has been put into place, it is loaded progressively (e.g. by placing slabs thereon). It is during this loading step that the tensioners are progressively put under tension, so as to compress and bend the beams as they are being put under load.

That technique makes it possible to compensate progressively for the forces exerted on the beam while it is being loaded. It thus makes it possible to reach the limits of the concrete's ability to withstand compression, so much so that a beam obtained by this technique may be subjected to greater loading than a beam obtained by pre-tensioning.

However, that relatively complex technique is generally restricted to large works, since it requires bulky tensioning machines to be used. Furthermore, it turns out to be expensive, not only because of the use of such machines, but also because the tensioners are passed through special and expensive sheaths in which a special coating is injected to make it possible to transmit forces to the beam.

OBJECT OF THE INVENTION

In order to remedy the above-mentioned drawbacks of the prior art, the present invention proposes a novel structural element that presents the advantages of pre-tensioning, and that is adapted to support greater loads.

More particularly, the invention proposes a structural element as defined in the introduction, in which at least one second tensioner is provided that is fastened to said elongate body at two distinct points so that it compresses and bends the elongate body, and in which deactivation means are provided for relaxing the compression and the bending exerted on the elongate body by said second tensioner.

The first and second tensioners are thus put under tension while casting the body (beam, slab, . . . ) of the structural element in a factory. The method of fabricating the structural element is thus as easy to implement as the pre-tensioning method.

The first and second tensioners are generally situated longitudinally in the body, along two of its opposite faces. In this way, the tension exerted on the first and second tensioners generates little, if any, bending of the structural element (the hogging generated by the first tensioner is compensated for by the sagging generated by the second tensioner).

Thus, it can be understood that the structural element could be unmolded sooner, since there is no risk of cracking when relaxing the tensioners and unmolding. Fabrication of the element is thus quicker, which is beneficial to its cost of production.

By means of the invention, the second tensioner(s) make(s) it possible to compress and to bend the body in provisional manner. After relaxing the second tensioners, the structural element behaves as a structural element obtained by a pre-tensioning method.

The major advantage of the invention is that, since the body bends little, if at all, it is possible to apply more compression to it.

Specifically, so long as the second tensioners are tensioned, the structural element bends little, if at all. It could thus be envisaged to relax the second tensioners progressively while the structural element is being loaded, so that by relaxing the second tensioners in succession, it is possible to compensate for the deformation of the structural element caused by its load.

Consequently, since there is less bending, there is less risk of cracks appearing, such that it is possible to apply greater tension to the first tensioners (and to the second tensioners), to come close to the compression limits of the concrete.

It is also possible to space the first tensioners as far as possible from the neutral axis of the body, so that after the second tensioners have been removed from the body, the first tensioners exert a large bending moment on the body that opposes the bending generated by the body's own weight and by the loads it supports.

As a result of the greater prestress, the structural element is thus capable of supporting heavier loads.

Alternatively, in order to support a load of the same weight, it is possible to reduce the section of the beam, which would lead to savings in weight, bulkiness, and cost.

The structural element of the invention has other characteristics that are advantageous and non-limiting and that are as follows:

-   -   the second tensioner is fastened to said elongate body by         fastener means, and at least one of said fastener means includes         a removable portion that forms said deactivation means;     -   the second tensioner is made of two portions that are situated         in alignment with each other and that are connected together by         fastener means, and the fastener means include a removable         portion that forms said deactivation means;     -   said removable portion is meltable;     -   said removable portion is detachable;     -   at least a central portion of the second tensioner is accessible         from the outside of the elongate body in order to be cut or         broken;     -   the second tensioner is fastened to said elongate body via its         ends, and the deactivation means are adapted solely to relax the         compression and the bending exerted on the elongate body by the         central portion of said second tensioner;     -   each second tensioner is adapted to be removed from said         elongate body completely or in part;     -   each second tensioner comprises a metal bar, cable, or wire;     -   at least a central portion of the second tensioner is passed         freely into a sheath that is fastened to the elongate body;     -   at least a central portion of the second tensioner is situated         outside the elongate body;     -   each first tensioner comprises a metal bar, cable, or wire that         is embedded in the material of the elongate body; and     -   the body is formed by a concrete beam or by a concrete slab.

The invention also proposes a method of building a work, the method comprising steps of:

a) installing an initial structure;

b) fastening a structural element as mentioned above to said initial structure;

c) installing a subsequent structure on said structural element, so that said structural element is put under load; and

d) deactivating each second tensioner of said structural element so as to relax the compression and the bending exerted by the second tensioner on the elongate body of said structural element.

Preferably, steps c) and d) are performed in concurrent manner.

Advantageously, said subsequent structure comprises at least one structural element of the invention, and steps c) and d) are followed by steps of:

e) adding a screed or concrete topping or a permanent heavy load on said subsequent structure; and

f) deactivating each second tensioner of the structural element of said subsequent structure so as to relax the compression and the bending exerted by the second tensioner on the elongate body of said structural element.

DETAILED DESCRIPTION OF AN EMBODIMENT

The following description of non-limiting examples given with reference to the accompanying drawings, makes it possible to understand what the invention consists of and how it can be reduced to practice.

In the accompanying drawings:

FIGS. 1A and 1B are diagrammatic side views in section of a beam of rectangular section of the invention;

FIG. 1C is a diagram of the forces and moments exerted with no load on the body of the beam shown in FIGS. 1A and 1B;

FIG. 1D is a diagram of the forces and moments exerted under load on the body of the beam shown in FIGS. 1A and 1B;

FIGS. 2A, 2B, and 2C are section views of a detail of three embodiments of the deactivation means for deactivating a second tensioner of the beam shown in FIGS. 1A and 1B;

FIGS. 3A and 3B are diagrammatic side views in section of an I-section beam of the invention;

FIGS. 4A and 4B are a diagrammatic side view and section view of a slab of the invention; and

FIGS. 5A and 5B are diagrammatic views of a work making use of beams and slabs of the invention.

Large-size works (buildings, bridges) are generally made by means of prefabricated structural elements that are assembled together on a building site.

In the description, the terms “bottom” and “top” are used relative to the work once assembled, the bottom portion of an element designating the portion of the element that is situated closer to the ground, and the top portion designating the portion of the element that is situated on the side opposite therefrom.

FIGS. 1A, 3A, 4A, and 5A show four types of prefabricated structural element 10, 20, 30, 40. In FIGS. 1A and 3A, the structural elements are beams 10, 20. In FIGS. 4A and 5A, the structural elements are slabs 30, 40.

The points that are common to the various structural elements 10, 20, 30, 40 are described together in a first portion of the description. The structural elements 10, 20, 30, 40 are then described successively in detail in a second portion of the description.

Each structural element 10, 20, 30, 40 firstly includes an elongate body 11, 21, 31, 41. The body gives the structural element 10, 20, 30, 40 its general shape. It also incorporates reinforcement serving to prestress it.

It is assumed herein that the body 11, 21, 31, 41 is made of concrete. In a variant, some other material could be used. It is also assumed that the body 11, 21, 31, 41 is designed to be placed horizontally in the work, so as to be fastened therein via its two ends, and so as to be loaded (i.e. so as to support heavy elements).

It should be understood that the structural element 10, 20, 30, 40 under consideration is thus subjected to two forces: its own weight and the weight of the load. Usually, the load can be subdivided into two components: a permanent component and a variable component.

Under the effect of these two forces, the body 11, 21, 31, 41 naturally tends to deform, bending downwards (the body is said to “sag”). This bending generates compression stresses in the top portion of the body, and traction stresses in the bottom portion of the body. As a result of the traction or compression stresses, there is a risk of the concrete cracking in the bottom face or in the top face of the body.

In order to avoid the appearance of such cracks, each of the structural elements 10, 20, 30, 40 is provided with at least one first tensioner 1 that is fastened in the body 11, 21, 31, 41 in such a manner that said body becomes compressed.

The compression obtained thus makes it possible to compensate for the above-mentioned traction stresses, so as to avoid cracks appearing. The body 11, 21, 31, 41 is thus prestressed in this way by each first tensioner 1.

Generally, a plurality of rectilinear first tensioners 1 are used. The first tensioners 1 could be formed by wires, by metal cables, or by high-capacity steel bars. Preferably, they are embedded in the concrete in such a manner as to transmit forces to the body of the structural element 10, 20, 30, 40 progressively. In addition, they are off-center relative to the neutral axis A1 of the body 11, 21, 31, 41 of the structural element 10, 20, 30, 40 under consideration.

As shown clearly in FIGS. 1B, 3B, 4B, and 5, the first tensioners 1 are preferably distributed below the neutral axis A1 of the body 11, 21, 31, 41 of the structural element 10, 20, 30, 40 under consideration.

As shown clearly in FIGS. 1A and 1B (and the same applies for the other structural elements 20, 30, 40), the first tensioners 1 extend lengthwise parallel to the neutral axis A1 of the body 11 of the beam 10, and they are regularly distributed on either side of a bottom mean axis A2.

The total traction exerted on the first tensioners 1 thus makes it possible to compress the body 11 of the beam 10 along the bottom mean axis A2.

As shown in FIG. 1C, the first tensioners 1 thus exert a compression force E1 on the body 11 of the beam 10. The compression force E1 generates compression stresses that are distributed in uniform manner over the entire section of the body 11.

As shown in FIG. 1A, the bottom mean axis A2 is situated below the neutral axis A1, at a distance from said neutral axis referenced offset D1.

As shown in FIG. 1C, as a result of the offset D1, the first tensioners 1 exert a bending moment M1 on the body 11 of the beam 10. The bending moment M1 causes the body 11 to bend upwards, i.e. in a direction that is opposite to the direction in which it tends to bend under the effect of its own weight (the beam is thus said to present a negative sag known as “hogging”). The bending moment M1 generates traction stresses in the top portion of the body 11. It also generates compression stresses in the bottom portion of the body 11.

When the body 11, 21, 31, 41 of the structural element 10, 20, 30, 40 is in place in the work and is loaded, the bending moment M1 serves to compensate for the bending of the body under the effect of its own weight and under the effect of the load that it carries.

In contrast, before the structural element 10, 20, 30, 40 is put into place in the work, the bending moment M1 is not compensated for, and therefore tends to bend the body 11, 21, 31, 41 (in particular when the structural element is stored vertically, and when its own weight no longer causes the body to bend downwards).

In practice, while the offset D1 and/or while the traction force exerted on the first tensioners 1 remain(s) less than a threshold, the traction stresses generated by the bending moment M1 are compensated for by the compression stresses generated by the compression force E1: the stresses remain within acceptable limits. There is thus no risk of cracking.

The drawback is then that the bending moment M1 remains limited as a result of the offset D1, to such an extent that the loads that the structural element 10, 20, 30, 40 can support are also limited.

The invention thus proposes a method that makes it possible to increase the offset D1 and/or to increase the forces exerted on the first tensioners so as to be able to apply a greater load to the structural element 10, 20, 30, 40, but without the risk of cracks appearing in the concrete.

Thus, according to a particularly advantageous characteristic of the invention, the structural element 10, 20, 30, 40 includes at least one second tensioner 2 that is fastened to the body 11, 21, 31, 41 at two distinct points by two fastener means 3 so that it compresses and bends the body 11, 21, 31, 41 downwards, and deactivation means 3B are provided for relaxing the compression and the bending exerted on the elongate body 11, 21, 31, 41 by the second tensioner 2.

In practice, each second tensioner 2 is situated on the body 11, 21, 31, 41 in off-center manner relative to the neutral axis A1, so that when the structural element has not yet been put into place in the work and is not yet loaded, the compression that each second tensioner exerts on the structural element serves to avoid cracks appearing.

Preferably, each second tensioner 2 is situated above the neutral axis A1 of the body 11, 21, 31, 41 in such a manner that it exerts a bending moment on the body that opposes the bending moment M1, at least in part.

Generally, a plurality of rectilinear second tensioners 2 are used. The second tensioners 2 could be formed by wires, by metal cables, or by metal bars, preferably made of a suitable steel.

As shown clearly in FIGS. 1B, 3B, 4B, and 5, the second tensioners 2 are preferably distributed above the neutral axis A1 of the body 11, 21, 31, 41 of the structural element 10, 20, 30, 40 under consideration.

As shown in FIGS. 1A and 1B, for example, (and the same applies for the other structural elements 20, 30, 40), the second tensioners 2 are elongate parallel to the neutral axis A1 of the body 11 of the beam 10, and they are regularly distributed on either side of a top mean axis A3.

The total traction exerted on the second tensioners 2 thus makes it possible to compress the body 11 of the beam 10 along the top mean axis A3.

As shown in FIG. 1C, the second tensioners 2 exert a compression force E2 on the body 11 of the beam 10 that is added to the compression force E1. The compression force E2 generates compression stresses that are distributed in uniform manner over the entire section of the body 11.

As shown in FIGS. 1A and 1B, for example, the top mean axis A3 is situated above the neutral axis A1, at a distance from said neutral axis referenced offset D2.

As shown in FIG. 1C, as a result of the offset D2, the second tensioners 2 exert a bending moment M2 on the body 11 of the beam 10, in the opposite direction to the bending moment M1. The bending moment M2 causes the body 11 to bend downwards, so much so that it makes it possible to compensate, at least in part, for the bending of the body 11 under the effect of the bending moment M1.

In other words, the bending moment M2 serves to simulate a load on the beam 10 when said beam is not yet loaded. When the beam 10 begins to be loaded, the deactivation means of each second tensioner 2 then make it possible to relax the second tensioner 2 so as to eliminate the bending moment M2.

By means of the invention, it is thus possible to maximize the offsets D1 and D2 and the traction forces exerted on the first and second tensioners 1, 2, while avoiding the body bending too much when said body is not yet loaded, which avoids cracks appearing in the concrete and which avoids exceeding the acceptable compression limits of the concrete.

Maximizing the offsets D1 and D2 and the traction forces thus makes it possible to increase the values of the bending moments M1 and M2. In this way, when the second tensioners 2 are removed from the body 11, 21, 31, 41 of the structural element (while the structural element is being loaded progressively), the bending moment M1 enables the structural element 10, 20, 30, 40 to support loads that are heavier than the loads that it would have supported if the offset D1 or if the force exerted on the first tensioners 1 had been smaller.

The only limit on this method remains the limiting compression that the concrete can withstand. This is why more emphasis is put on maximizing the offsets D1 and D2 than on increasing the traction forces exerted on the first and second tensioners 1, 2.

In FIG. 1C, the sum of the forces and the moments exerted by the first and second tensioners 1, 2 is referenced C1.

As shown in FIG. 1D, when the structural element 10, 20, 30, 40 is put under load, the load and the structural element's own weight exert a bending moment M3 that is added to the above-mentioned sum C1.

By relaxing the second tensioners 2, it is thus possible to eliminate the compression force E2 and the bending moment M2 previously exerted on the structural element, so as to compensate, at least in part, for the bending moment M3.

Thus, the sum of the forces and the moments exerted on the structural element, referenced C2, makes it possible to ensure that:

-   -   no traction stress appears in the structural element; and     -   the compression stresses to which the structural element is         subjected remain less than the compression limits of the         concrete.

In a variant of the invention in which the ends of the second tensioners are fastened in non-detachable manner to the body of the structural element, provision could be made to relax only the central portions of the second tensioners. By way of example, provision could thus be made to cut the second tensioners at their centers, so that their two ends remain fastened to the body of the structural element. In this variant, after the second tensioners have been cut, their ends thus continue to exert compression forces and bending moments at the ends of the body of the structural element. These forces and moments thus make it possible to compensate for the forces that the first tensioners exert on the ends.

The particular embodiment of the rectangular-section beam shown in FIGS. 1A and 1B is described below in greater detail.

In this embodiment, the body 11 of the beam 10 presents a shape that is substantially a rectangular parallelepiped.

The first tensioners 1 are embedded completely in the concrete of the body 11 of the beam 10, with the possible exception of their ends that may project from the body. They are thus not removable from the body of the beam 10.

The second tensioners 2 are slidably mounted in the body 11 of the beam 10. As shown in FIG. 2B, the second tensioners 2 are passed for this purpose into sheaths 4 that are embedded in the concrete in such a manner that their ends open out into both ends of the body 11. The sheaths 4, which are made of plastics material in this embodiment, avoid the concrete bonding to the second tensioners 2.

As described above, provision could alternatively be made to embed the ends of the second tensioners in the ends of the body of the structural element.

In a variant, as shown in FIG. 2A, a sheath need not be used, with provision being made to move the second tensioners 2 (e.g. by pivoting them about their own axes) while the concrete is curing, so that the concrete does not adhere to the second tensioners 2. Still in a variant, a substance could be applied to the second tensioners that prevents the concrete from adhering. Provision could also be made to position the second tensioners outside the concrete, e.g. as described with reference to FIG. 4A.

As shown in FIG. 1B, in the previously-described embodiment, the first tensioners 1 are distributed over three rows and five columns. The first tensioner 1 that is situated at the center of the matrix thus extends along the bottom mean axis A2.

In this embodiment, there are half as many second tensioners 2 as there are first tensioners 1, and they are distributed relative to one another in substantially in the same way as the first tensioners. The second tensioner 2 that is situated at the center of the matrix thus extends along the top mean axis A3.

The neutral axis A1 that passes through the centers of the cross-sections of the body 11 thus extends between the top and bottom mean axes A3, A2, at equal distances therefrom.

As described above, each second tensioner 2 is fastened to the body 11 of the beam 10 at two distinct points by two fastener means, and deactivation means are provided for relaxing each second tensioner 2.

In practice, each second tensioner 2 is fastened via its two ends 2A to the two ends of the body 11 of the beam 10. In this way, the ends 2A remain easily accessible from outside the body 11.

The deactivation means are designed to make it possible, on site, to relax the tension of the second tensioner 2.

The deactivation means could be of various forms.

They could thus be incorporated with at least one of the fastener means 3 for fastening the ends 2A of the second tensioners 2 to the body 11 of the beam 10.

As shown in FIG. 2A, the fastener means 3 comprise a bushing 3A that is pressed against the corresponding end of the body 11 of the beam 10 and that houses two keys 3B internally.

The bushing 3A presents an outside face that is circularly cylindrical, and an inside face that is frustoconical having its apex directed towards the body 11.

Together, the keys 2B generally present the shape of a cylinder that is cut in two lengthwise. Each of them presents an outside face that is frustoconical, of a shape that matches the shape of the inside face of the bushing 3A, and an inside face that is notched.

The two keys 2B thus form a kind of chuck that, when they are driven into the bushing 3A towards the body 11, make it possible to block the end 2A of the second tensioner 2 securely.

In the embodiment shown in FIG. 2A, the above-mentioned deactivation means are thus formed by the two keys 3B. Specifically, the two keys 3B project out from the bushing 3A, in such a manner that they are adapted to be pulled mechanically out from the bushing 3A so as to relax the end 2A of the second tensioner 2.

In the embodiment shown in FIG. 2B, the deactivation means are meltable. In this configuration, the deactivation means are formed by a layer of metal or by a non-metallic paste that covers the end 2A of the second tensioner 2 (or the inside face(s) of the bushing 3A or of the keys 3B), having a melting temperature that is relatively low, and that presents mechanical characteristics that are satisfactory. The deactivation means could thus be a layer of zinc or of tin, since the melting temperature of these materials is quite low (less than 450° C., preferably about 200° C. to 300° C.) so as to enable it to be melted on site, and since its stiffness at ambient temperature is sufficient to bond well to the second tensioner 2.

In a variant, the removable fastener means could be in some other form.

They could thus be in the form of a threaded sleeve that is fastened on the second tensioner, and that is screw-fastened in a nut that is engaged in the end of the sheath 4. In order to relax the compression exerted by the second tensioner on the body of the beam, it thus suffices to unscrew the threaded sleeve so that it escapes from the nut.

In another variant, the fastener means used could be in the form of an adhesive or of a meltable paste. They could also be in the form of a sleeve that can be destroyed easily (e.g. by cutting), that is passed onto the second tensioner, and that bears against the body.

Still in a variant, the end 2A of the second tensioner 2 could be coated with a layer of zinc or of tin and could be embedded directly in the concrete. Thus, by heating the layer of metal, it is possible to melt it so that the second tensioner 2 can escape from the concrete and relax the compression and the bending that it exerts on the body 11 of the beam 10.

In any event, second fastener means, designed to block the other end of the second tensioner 2, could also be in various forms.

Preferably, they are removable. In this way, the second tensioner 2 could be removed from the body 11 of the beam 10, and it could be reused on other beams, which reduces costs.

They could thus be in the same form as the removable first fastener means. They could also be in the form of a sleeve that is passed and fastened onto the second tensioner, and that merely comes to bear against the body.

In a variant, the second fastener means could fasten the second end of the second tensioner 2 to the body 11 of the beam 10 in non-removable manner, so that the second tensioner 2 cannot be removed from the body 11. Thus, provision could be made for the second end of the second tensioner 2 to be embedded in the concrete of the body 11.

As shown in FIG. 2C, in a variant, provision could be made for the deactivation means to be situated not at one of the ends of the second tensioner 2, but at a distance from the ends. In this variant, the ends 2A of the second tensioner 2 could be blocked in the body 11 of the beam 10 in (optionally) non-removable manner.

In this variant, the second tensioner 2 is made of two portions 2C, 2D that are situated in alignment with each other, and that are connected together by fastener means 5.

The fastener means 5 comprise a sleeve 5A inside which two pairs of keys 3B are housed. The keys 3B are identical to the keys shown in FIG. 2A. The sleeve 5A presents an inside face that presents two frustoconical portions that face in opposite directions.

In this embodiment, each of the touching ends of the two portions 2C, 2D of the tensioner are coated with a coating of zinc or of tin, or with another material that presents suitable characteristics, and in which a resistance wire runs.

When the second tensioner 2 is put under traction (during casting of the body 11 of the beam 10), the keys 3B move towards the two ends of the sleeve 5A, which enables them to close like two jaws on the touching ends of the two portions 2C, 2D of the second tensioner 2.

In order to deactivate the second tensioner 2, it is thus necessary to supply the resistance wire with electric current (from outside the body 11 of the beam 10), so that the coating melts and so that the touching ends of the two portions 2C, 2D of the second tensioner escape from the sleeve 5A.

A specific example of a beam 10 that can be used on site is given below.

The beam could thus present a length of 7 meters (m), a height L2 of 60 centimeters (cm), and a width L3 of 40 cm.

The first tensioners 1 could be distributed in such a manner that the bottom mean axis A2 extends at 6.8 cm from the bottom face of the body 11 of the beam 10.

The second tensioners 2 could be distributed in such a manner that the top mean axis A3 extends at 5 cm from the top face of the body 11 of the beam 10.

The beam 10 could be prestressed by means of the first tensioners 1 with a compression force E1 that is equal to 192 (metric) tonnes (t).

The beam 10 could also be prestressed in provisional manner by means of the second tensioners 2 with a compression force E2 that is equal to 120 t.

FIG. 3A shows a second embodiment of a beam 20 of the invention.

In this embodiment, the body 21 of the beam 20 presents a cross-section that is I-shaped.

As shown in FIG. 3B, the body 21 of the beam 20 thus presents two parallel flanges 23 between which a vertical web 22 extends.

The first and second tensioners 1, 2 thus extend over the entire length of the beam 20, parallel to one another.

In this embodiment, five first tensioners 1 are provided, embedded in the concrete of the bottom flange 23 of the body 21 of the beam 20. In this embodiment, the first tensioners 1 are once more regularly distributed over the width of the beam 20. Thus, when they compress the body 21 of the beam 10, they do not deform it in twisting.

The five first tensioners 1 are situated in the proximity of the bottom face of the bottom flange 23. Thus, when they are tensioned, they make it possible to bend the body 21 of the beam 20 so that the center of the beam moves upwards.

Three second tensioners 2 are also provided that are regularly distributed over the width of the beam 20. The fastener means for fastening the ends of the second tensioners 2 to the body 21 of the beam 20 are identical to the fastener means described with reference to FIG. 2B.

FIGS. 4A and 4B show a first embodiment of a slab 30 of the invention.

In this embodiment, the body 31 of the slab 30 presents the general shape of a rectangular parallelepiped. Its neutral axis A1 thus coincides with the central longitudinal axis of the body 31.

In this embodiment, the ends of the slab 30 nevertheless present respective rims 32 projecting from the top face of the body 31. The two rims 32 run along the two ends of the body 31. They co-operate with each other to define a cavity 33 between them into which a screed could be poured.

The first and second tensioners 1, 2 extend over the entire length of the slab 30, parallel to one another.

In this embodiment, a plurality of first tensioners 1 are provided, embedded in the concrete of the body 31 of the slab 30. In this embodiment, the first tensioners 1 are once more regularly distributed over the width of the slab 30, below the neutral axis A1. They are situated in the proximity of the bottom face of the body 31 of the slab 30.

Second tensioners 2 are also provided that are regularly distributed over the width of the slab 30, above the neutral axis A1. The second tensioners 2 pass through the two rims 32 of the body 31 of the slab 30 so that their ends project from either end of the body 31 of the slab 30. In contrast, a central portion of each of the second tensioners 2, that extends between the two rims 32, is situated in the cavity 33, outside the body 31.

In this embodiment, the second tensioners 2 are sheathed over their entire length, which ensures that they slide through the rims 32, and which guarantees that they are protected from the atmosphere. The sheaths are also useful when a concrete screed is poured into the cavity 33, since they avoid the concrete of the screed adhering to the second tensioners 2.

In a variant, it could be envisaged that the second tensioners 2 are sheathed over a portion only of their length, the portion that is situated in the cavity 33. The portions of the second tensioners that pass through the rims 32 are coated with zinc or with tin, and they are embedded in the concrete. It order to enable the forces exerted by the second tensioners to be relaxed, it thus suffices to heat the zinc or tin coating.

In a variant, it could also be envisaged that the second tensioners 2 are not sheathed, when the height of the concrete screed, once poured into the cavity 33, is sufficiently small for the second tensioners to be situated above the screed.

The rims 32 make it possible to offset the second tensioners 2 by a large distance from the neutral axis A1 of the body 31. In this way, the traction force exerted on each of the second tensioners 2 may be smaller than the traction force exerted on the first tensioners 1. In this embodiment, it is thus possible to use second tensioners 2 of diameter that is smaller than the diameter of the first tensioners 1, or to reduce the number of second tensioners 2 that are used.

In this embodiment, the fastener means that are provided at the ends of the second tensioners 2 are also identical to the fastener means described with reference to FIGS. 1A, 1B, and 2B.

In a variant embodiment of the slab, it could be envisaged that the body (31) does not have a rim (32), so the second tensioners are situated entirely through the body of the slab.

FIG. 5A shows a second embodiment of slabs 40 of the invention. The slabs 40 shown in FIG. 5A differ from the slab 30 shown in FIGS. 4A and 4B only by the hollow feature of their bodies 41.

Specifically, it is known to use hollow slabs 40, i.e. slabs 40 having a body that is hollowed out with longitudinal ducts referred to as cells. The cells make it possible to reduce the weight of the slab while preserving its thickness, so as to ensure that it is very rigid.

However, in this embodiment, as shown in FIGS. 4A and 4B, it is preferable to use a slab 30 having a body 31 that is solid (as opposed to hollow).

Specifically, for a given section of slab, the solid surface area of the section of the slab is greater as a result of the absence of cells. Consequently, the slab is capable of being subjected to greater compression forces.

It is thus possible to exert higher forces on the first and second tensioners 1, 2, such that the slab 30 is capable of supporting greater bending moments.

A specific example of a slab 30, 40 that can be used on site is given below.

The slab could thus present a length of 9 m, a height L2 of 20 cm, and a width L3 of 1.2 m.

The first tensioners 1 could be distributed in such a manner that the bottom mean axis A2 extends at 4 cm from the bottom face of the body 31 of the slab 30, 40.

The second tensioners 2 could be distributed in such a manner that the top mean axis A3 extends at 4 cm above the top face of the body 31 of the slab 30, 40.

Consideration is give firstly to when the body 41 of the slab 40 is hollow (FIG. 5).

The slab 40 could be prestressed by means of the first tensioners 1 with a compression force that is equal to 140 t. The slab 40 could also be prestressed in provisional manner by means of the second tensioners 2 with a compression force that is equal to 60 t.

The result is that the hollow slab 40 can support loads that are twice as heavy as a hollow slab that has not been fitted with second tensioners 2. Specifically, it is designed to support a prestress of 140 t, which is much greater than the prestress that can be applied in the absence of second tensioners (which is about 82 t).

Consideration is give below to the slab 30 having a body 31 that is solid (FIGS. 4A and 4B).

The slab 30 can be prestressed by means of the first tensioners 1 with a compression force that is equal to 192 t. The slab 30 can also be prestressed in provisional manner by means of the second tensioners 2 with a compression force that is equal to 96 t.

As a result, the solid slab 30 can support loads that are three times heavier than a hollow slab that has not been fitted with second tensioners 2. Specifically, it should be understood that a solid slab can accommodate more prestress than a hollow slab.

There follows a description of how to use the above-mentioned beams 10 and slabs 40 given with reference to FIGS. 5A and 5B.

The beams 10 and slabs 40 are prefabricated in a factory.

Their fabrication process consists in: putting the first and second tensioners 1, 2 in molds (the second tensioners already being sheathed and fitted with their frustoconical sleeves 3); applying tension to the tensioners; pouring concrete into the molds; and waiting for the concrete to set completely. After the concrete has cured, the first and second tensioners 1, 2 are released, thus putting the bodies 11, 31 of the beams and slabs in off-center longitudinal compression.

The beams 10 and slabs 40 are then removed from their respective molds. As a result of the forces exerted by the first and second tensioners 1, 2, the bodies 11, 31 do not tend to curve excessively. Consequently, removing the beams 10 and the slabs 40 from their molds does not cause the bodies to bend suddenly, which avoids cracks appearing in the concrete.

It is assumed below that an initial structure of the work has already been assembled on the building site. Specifically, vertical supports 50 have already been installed spaced apart from one another.

The first operation thus consists in installing two beams 10 in parallel and spaced apart from each other, each extending between two supports 50 that are spaced apart from each other. To do this, the ends of each beam 10 are placed on the rims 51 provided on the supports 50.

The second operation consists in progressively loading the two beams 10 by installing slabs 40 thereon, the ends of each slab 40 bearing respectively on the two beams 10.

Putting the slabs progressively into place on the beams 10 has the effect of gradually urging the beams 10 downwards. In order to impede this bending phenomenon, the workers can progressively release the second tensioners 2 of the beams 10 so that said tensioners cease to exert a downward bending moment on the bodies 11 of the beams 10. It can thus be envisaged that the second tensioners 2 are released as the beams 10 are loaded, so that the beams 10 always preserve the best shape (substantially rectilinear).

Once all of the slabs 40 have been put into place, it can be envisaged to cover said slabs with a concrete screed (also referred to as “concrete topping”).

Prior to this operation, all or some of the second tensioners 2 present in the slabs 40 can be removed. When it is desired to remove some of the second tensioners 2 at the time of pouring the screed or after the concrete has set, boxes could thus advantageously be used at the ends of the second tensioners 2 in order to preserve access to the ends.

The present invention is not limited to the embodiments described and shown, and the person skilled in the art can apply any variation thereto in accordance with the spirit of the invention.

In particular, when the slabs used are hollow, it can be envisaged to put the second tensioners in the cells themselves, so as to make it easier to install them and to remove them.

In another variant of the invention that is not shown, in order to build a bridge, a beam of the type shown in FIGS. 3A and 3B could be used, with a length of 40 m, a section of 2.5 m in height, a bottom heel of 70 cm in width, a top heel of 1.2 m in width, and a core thickness of 24 cm. Relaxing the second tensioners makes it possible to obtain an effect similar to a second stage of post-tensioning. 

1-16. (canceled)
 17. A prefabricated structural element (10, 20, 30, 40) comprising: an elongate body (11, 21, 31, 41); and at least one first tensioner (1) that is fastened in the elongate body (11, 21, 31, 41) in such a manner that it compresses and bends the elongate body (11, 21, 31, 41) in a first direction; the structural element being characterized in that it includes at least one second tensioner (2) that is fastened to said elongate body (11, 21, 31, 41) at two distinct points so that it compresses and bends the elongate body (11, 21, 31, 41) in a direction that is opposite to said first direction; and in that deactivation means (3B) are provided for relaxing the compression and the bending exerted on the elongate body (11, 21, 31, 41) by said second tensioner (2).
 18. A structural element (10, 20, 30, 40) according to claim 17, wherein the elongate body (11, 21, 31, 41) comprises two ends and each second tensioner (2) comprises two ends embedded in the ends of the elongate body (11, 21, 31, 41).
 19. A structural element (10, 20, 30, 40) according to claim 17, wherein the second tensioner (2) is made of two portions (2C, 2D) that are situated in alignment with each other and that are connected together by fastener means (5), and the fastener means (5) include a removable portion (3B; 3C) that forms said deactivation means.
 20. A structural element (10, 20, 30, 40) according to claim 17, wherein the second tensioner (2) is fastened to said elongate body (11, 21, 31, 41) by fastener means (3), and at least one of said fastener means (3) includes a removable portion (3B; 3C) that forms said deactivation means.
 21. A structural element (10, 20, 30, 40) according to claim 18, wherein said removable portion (3C) is meltable.
 22. A structural element (10, 20, 30, 40) according to claim 19, wherein said removable portion (3C) is meltable.
 23. A structural element (10, 20, 30, 40) according to claim 18, wherein said removable portion (3B) is detachable.
 24. A structural element (10, 20, 30, 40) according to claim 19 wherein said removable portion (3B) is detachable.
 25. A structural element according to claim 17, wherein at least a central portion of the second tensioner is accessible from the outside of the elongate body in order to be cut or broken.
 26. A structural element according to claim 17, wherein the second tensioner is fastened to said elongate body via its ends, and the deactivation means are adapted solely to relax the compression and the bending exerted on the elongate body by the central portion of said second tensioner.
 27. A structural element (10, 20, 30, 40) according to claim 17, wherein each second tensioner (2) is adapted to be removed from said elongate body (11, 21, 31, 41) completely or in part.
 28. A structural element (10, 20, 30, 40) according to claim 17, wherein each second tensioner (2) comprises a metal bar, cable, or wire.
 29. A structural element (10, 20, 30, 40) according to claim 17, wherein at least a central portion of the second tensioner (2) is passed freely into a sheath (4) that is fastened to the elongate body (11, 21, 31, 41).
 30. A structural element (10, 20, 30, 40) according to claim 17, wherein at least a central portion of the second tensioner (2) is situated outside the elongate body (11, 21, 31, 41).
 31. A structural element (10, 20, 30, 40) according to claim 17, wherein each first tensioner (1) comprises a metal bar, cable, or wire that is embedded in the material of the elongate body (11, 21, 31, 41).
 32. A structural element (10, 20, 30, 40) according to claim 17, wherein the body is formed by a concrete beam (11, 21) or by a concrete slab (31, 41).
 33. A method of building a work, the method comprising steps of: a) installing an initial structure (50); b) fastening a structural element (20) in accordance with claim 1 to said initial structure (50), each first tensioner (1) being, at this step, fastened in the elongate body (11, 21, 31, 41) in such a manner that it compresses and bends the elongate body (11, 21, 31, 41) in a first direction, and each second tensioner (2) of the structural element (20) being fastened to said elongate body (11, 21, 31, 41) at two distinct points so that it compresses and bends the elongate body (11, 21, 31, 41) in a direction that is opposite to said first direction; c) installing a subsequent structure (40) on said structural element (20), so that said structural element (20) is put under load; and d) deactivating each second tensioner (2) of said structural element (20) so as to relax the compression and the bending exerted by the second tensioner (2) on the elongate body (21) of said structural element (20).
 34. A method according to claim 33, wherein steps c) and d) are performed in concurrent manner.
 35. A method according to claim 33, wherein said subsequent structure comprises at least one prefabricated structural element (10, 20, 30, 40) comprising: an elongate body (11, 21, 31, 41); and at least one first tensioner (1) that is fastened in the elongate body (11, 21, 31, 41) in such a manner that it compresses and bends the elongate body (11, 21, 31, 41) in a first direction; the structural element being characterized in that it includes at least one second tensioner (2) that is fastened to said elongate body (11, 21, 31, 41) at two distinct points so that it compresses and bends the elongate body (11, 21, 31, 41) in a direction that is opposite to said first direction; and in that deactivation means (3B) are provided for relaxing the compression and the bending exerted on the elongate body (11, 21, 31, 41) by said second tensioner (2), and steps c) and d) of the method are followed by steps of: e) adding a screed or concrete topping or a permanent heavy load on said subsequent structure; and f) deactivating each second tensioner (2) of said structural element (40) of said subsequent structure (40) so as to relax the compression and the bending exerted by the second tensioner (2) on the elongate body (41) of said structural element (40).
 36. A method according to claim 33, wherein said subsequent structure comprises at least one prefabricated structural element (10, 20, 30, 40) comprising: an elongate body (11, 21, 31, 41); and at least one first tensioner (1) that is fastened in the elongate body (11, 21, 31, 41) in such a manner that it compresses and bends the elongate body (11, 21, 31, 41) in a first direction; the structural element being characterized in that it includes at least one second tensioner (2) that is fastened to said elongate body (11, 21, 31, 41) at two distinct points so that it compresses and bends the elongate body (11, 21, 31, 41) in a direction that is opposite to said first direction; and in that deactivation means (3B) are provided for relaxing the compression and the bending exerted on the elongate body (11, 21, 31, 41) by said second tensioner (2), wherein the elongate body (11, 21, 31, 41) comprises two ends and each second tensioner (2) comprises two ends embedded in the ends of the elongate body (11, 21, 31, 41), and steps c) and d) of the method are followed by steps of: e) adding a screed or concrete topping or a permanent heavy load on said subsequent structure; and f) deactivating each second tensioner (2) of said structural element (40) of said subsequent structure (40) so as to relax the compression and the bending exerted by the second tensioner (2) on the elongate body (41) of said structural element (40). 